1. Field of the Invention
The present invention relates to a focus detecting apparatus which is suitable for, for example, focus detection of a photographing lens of a single-lens reflex camera or the like.
2. Related Background Art
In recent years, many photographing cameras or video cameras have built-in focus detecting apparatuses for automatic focus adjustment. In focus detecting apparatuses, such as a single-lens reflex camera, each requiring severe focus precision, a focus detecting apparatus using a so-called image blurring method (phase difference detecting method) is generally used (refer to Japanese Patent Application Laid-Open No. 2001-66496 for example).
FIG. 8 is a cross sectional view of a main portion of a conventional focus detecting apparatus using an image blurring method of this sort.
In the figure, reference symbol ο designates an object plane (subject plane), reference numeral 81 designates an objective lens, and reference numeral 82 designates a field lens. The field lens 82 is provided in the vicinity of a predetermined formation plane (a focal surface in a camera) of the objective lens 81. Reference numerals 84a and 84b designate secondary image forming lenses, respectively, which are disposed symmetrically with each other with an optical axis L of the objective lens 81 as a center. The secondary image forming lenses 84a and 84b form two object images based on light beams that have passed through two regions 81a and 81b having different pupils of the objective lens 81. Reference numeral 86 designates a photoelectric transducer having two pixel lines (line sensors) 86a and 86b each including a plurality of elements. Two object images are formed on the surfaces of the line sensors 86a and 86b by the secondary image forming lenses 84a and 84b, respectively. This focus detecting apparatus detects a focus state of the objective lens 81 based on a relative positional relationship between the object images (a plurality of light quantity distributions related to the object images). Each of the line sensors 86a and 86b shown in FIG. 8 is constituted by a charge coupled device (CCD) or the like for example.
Reference numeral 88 designates a mask which has opening portions 88a and 88b formed therein and which is provided in the vicinity of the secondary image forming lenses 84a and 84b. The field lens 82 has a function for forming images of the opening portions 88a and 88b of the mask 88 in the different pupil regions 81a and 81b of the objective lens 81, and causes a conjugate relationship to be established between the pupil regions 81a and 81b, and the opening portions 88a and 88b. Reference numeral 89 designates a visual field mask which is provided in order to limit a visual field to the vicinity of the predetermined image forming plane 83. Each of sizes of the secondary images which are formed on the surfaces of the line sensors 86a and 86b, respectively, is limited by the visual field mask 89 to prevent one secondary image on one line sensor from overlapping the other secondary image on the other line sensor adjacent thereto.
In such a focus detecting apparatus, when for example, the objective line 81 is moved toward an object side (a left side in the figure) to provide a so-called front focus state, the secondary images which are formed on light receiving surfaces of the line sensors 86a and 86b by the secondary image forming lenses 84a and 84b, respectively, are shifted in directions indicated by two arrows A. The focus detecting apparatus detects the front focus state and a quantity of displacement based on changes in outputs of the line sensors 86a and 86b corresponding to the relative displacement of those secondary images. On the other hand, in case of a rear focus state, the secondary images are shifted in directions opposite to the directions indicated by the arrows A in the case of the front focus state, respectively. Thus, the focus detecting apparatus detects the rear focus state and a quantity of displacement based on changes in outputs of the line sensors 86a and 86b corresponding to the relative displacement of those secondary images.
In recent years, the focus detection for a wider range, and the focus detection with higher precision have been required. Thus, the focus detecting apparatus of the conventional focus detecting system is difficult to respond to those requests. Hereinafter, a description will be given with respect to problems which arise when a focus detection range is increased or distance measurement precision is enhanced with reference to FIG. 9. In FIG. 9, the same constituent elements as those shown in FIG. 8 are designated with the same reference numerals.
In the figure, similarly to the conventional focus detecting apparatus shown in FIG. 8, the light beams which are based on the object image and which are emitted from a point O on the optical axis of the objective lens 81 form images at the center portions of the line sensors 86a and 86b, respectively. On the other hand, when attention is paid to the light beams which are based on the object image and which are emitted from a point Oa located off an optical axis, these light beams pass through the pupil region 81a of the objective lens 81 to form an image on a position 89a at the upper end of the visual field mask 89. Then, after being deflected by the field lens 82 to pass through the opening portion 88a of the mask 88, those light beams form an image on the surface of the line sensor 86 a through the secondary image forming lens 84a. At this time, the opening portion of the visual field mask 89 for limiting the focus detection region is so large that the light beams that have passed through the pupil region 81a of the objective lens 81 are not made incident to the line sensor 86a on which an image is to be essentially formed from those light beams, but are made incident to an upper end portion of the line sensor 86b adjacent to the line sensor 86a. Note that in case of the light beams as well emitted from a point Ob located outside the optical axis, there arises the same problem as that in the case of the light beams emitted from the point Oa located outside the optical axis.
FIG. 10 shows a state of an image (secondary image) which is formed on the surface of the line sensor 86 in the focus detecting system shown in FIG. 9. Since ranges each having an image formed therein as indicated by a dotted line in the figure overlap each other on the surfaces of the line sensors 86a and 86b, even when the line sensors 86a and 86b are simply extended, a correlation between the two images cannot be obtained. It is understand from this that even if the visual field mask 84 is only simply enlarged, the focus detection region cannot be increased.
In addition, in a case as well where a secondary image formation magnification, i.e., an image formation magnification between the image formed on the predetermined image formation place and the image formed on the surface of the line sensor 86 is intended to be increased in order to increase the focus detection precision, the image on the surface of the line sensor 86 is enlarged. Therefore, similarly to the case of increasing the detection region, there arises a problem in that the light beams are made incident to the surface of the adjacent line sensor to form an image thereon. When a size of the image on the surface of the line sensor is reduced, and the focus detection region is increased, the problem in that the light beams are made incident to the surface of the adjacent line sensor to form an image thereon is solved to some extent. However, there is a possibility that the reduction of the focus detection precision occurs.